Spectral sensitization techniques are well known for photographic materials, i.e., techniques of expanding the light sensitive wavelength region of a silver halide photographic emulsion to make the emulsion sensitive to light having a longer wavelength by adding a certain cyanine dye or dyes to the photographic emulsion. It is also known that the sensitivity obtained by a spectral sensitization ("spectral sensitivity") is influenced by the chemical structure of a sensitizing dye and various properties of the silver halide emulsion such as, for example, the halide composition of silver halide, the crystal habit and crystal system of silver halide, the silver halide concentration, the hydrogen ion concentration, etc. Furthermore, spectral sensitization is also influenced by photographic additives present in the silver halide emulsion, such as stabilizers, antifoggants, coating aids, precipitants, color couplers, hardening agents, etc.
In general, one sensitizing dye is used for sensitizing a specific spectral wave length region of a silver halide photographic emulsion. When a combination of two, three or more sensitizing dyes is used for sensitizing a silver halide emulsion, the sensitivity of the silver halide emulsion obtained is generally lower than that of a silver halide emulsion obtained by using each of the sensitizing dyes individually. However, the use of a combination of a certain sensitizing dye and one or more other sensitizing dyes sometimes super-additively and greatly increases the spectral sensitivity of a silver halide emulsion, a phonomenon known as a supersensitization. However, the sensitizing dye group which is selected to provide the combination for supersensitization is required to have a remarkable selectivity for each other and even a slight difference in chemical structures of sensitizing dyes greatly affects the supersensitizing action. Accordingly, the combination of sensitizing dyes exhibiting supersensitizing action cannot be predicted simply from the chemical structures of sensitizing dyes.
Various combinations of sensitizing dyes for increasing the sensitivity of silver halide photographic emulsions by such supersensitization have been reported. Of these combinations, the combination described in Japanese Patent Publication No. 50,324/83 is excellent for providing a silver halide photographic emulsion having a high sensitivity with reduced fog. However, when this combination is applied to obtaining recent super high-speed color photographic materials, the sensitivity obtained is insufficient.